1. Field of the Invention
The present invention generally relates to a charging-and-discharging device and a battery pack which is used for a portable-type electronic apparatus such as a note-type personal computer.
In a portable-type electronic apparatus such as a note-type personal computer, batteries are installed as a power source for the apparatus. In general, based on the operational cost of the apparatus and current capacity which can be discharged for an instant, secondary batteries such as NiCd batteries are installed. Many apparatuses include charging circuits to charge the built-in secondary batteries by easily connecting an AC adaptor to the apparatus.
In such apparatus, charging-and-discharging control technologies that achieve a parallel charging of a plurality of batteries, reduction of a power loss in the discharging, and a highly efficient use of the battery are desired.
2. Description of the Related Art
(1) Prior Art
A charging-and-discharging device having the above technologies comprises a voltage converter, an over-discharge prevention circuit, and a plural-battery-pack charging circuit. In the following, descriptions these circuits will be discussed in that order.
A. Voltage Converter
When the electronic apparatus is operated with the battery, voltage of the battery is usually decreased as the battery proceeds to discharge. Therefore, to maintain the voltage used in the electronic apparatus at a constant value, output of the battery is stabilized to the constant voltage by using a DC to DC converter. When the output of the battery is stabilized by using the DC to DC converter, there are two relationships between the voltage of the battery mounted in the electronic apparatus and the voltage used within a main body of the electronic apparatus.
One relationship is that the voltage of the battery is higher than the voltage used within the electronic apparatus. The voltage of the battery is lowered to the voltage used within the electronic apparatus by the DC to DC converter. This type of DC to DC converter is referred to a step-down-type converter.
The other relationship is that the voltage of the battery is lower than the voltage used within the electronic apparatus. The voltage of the battery is raised to the voltage used within the electronic apparatus by the DC to DC converter. This type of DC to DC converter is referred to a step-up-type converter.
In the electronic apparatus, use of the step-down-type converter and the step-up-type converter is determined according to power consumption of the apparatus, operation time of the apparatus with the secondary battery, size and weight of the apparatus, and so on.
In the following, the two types of DC to DC converters will be described.
Step-down-type Converter!
FIG. 1 shows a schematic diagram of a conventional charging-and-discharging device with a built-in charging device and the step-down-type converter. The device shown in FIG. 1 comprises a DC to DC converter 500, a charging control section 510 for controlling the charging, a DC connector 520 for connecting with the AC adaptor, etc., and a constant current circuit 530 for charging which constructs the charging device. The DC to DC converter 500 includes a DC to DC control section 501. The constant current circuit 530 includes a constant current control section 531. Numeral 540 indicates a secondary battery. Tr51 and Tr52 are transistors. D51-D55 are diodes. F51 and F52 are fuses. L51 and L52 are choke coils. C51 and C52 are capacitors. R51-R53 are resistors.
The DC connector 520 is used for supplying external power when operating an electronic apparatus with an external power source through the AC adaptor, or when charging the built-in secondary battery 540. The DC to DC converter 500 is used to generate the voltage required for the electronic apparatus from the external power source supplied through the DC to DC connector 520 or from the built-in secondary battery 540.
The constant current circuit 530 is supplied with the external power source through the DC connector 520, and generates power required for charging the secondary battery 540 when the charging control section 510 instructs the constant current circuit 530 to charge the secondary battery 540.
The charging control section 510 is a control section for charging the secondary battery 540. Instructions for starting, stopping, and completing the charging are supplied to the constant current circuit 530 by the charging control section 510.
When the AC adaptor, etc., is connected to the DC connector 520, if the AC adaptor is deactivated because AC power source is not supplied to the AC adaptor, the diode D51 prevents the power of the built-in secondary battery 540 from flowing out. The diode D51 is a back-current-block protection diode.
The diode D52 supplies the power from the secondary battery 540 to the DC to DC converter 500 when no external power source is provided. Further, when the external power source is supplied through the DC connector 520, the diode D52 is operative as a protection diode which prevents a voltage of the external power source from being applied to the secondary battery 540.
The diode D55 prevents a back current from the secondary battery 540 to the constant current circuit 530.
The DC to DC converter 500 and the constant current circuit 530 are respectively constructed with conventional switching type regulators, etc. For the constant current control section 531, for example, an IC MB3759 made by FUJITSU L.T.D. is usable. For the DC to DC control section 501, for example, an IC MB3776A made by FUJITSU L.T.D. is usable.
When the AC adaptor, etc., is connected to the DC connector 520 to supply the external power, the external power source is applied to the DC to DC converter 500 through the diode D51, the DC to DC converter 500 producing the voltage required for the electronic apparatus. In this case, the power supplied from the external power source is blocked by the diode D52 and is not supplied to the secondary battery 540.
When the external power source is supplied, when the constant current circuit 530 is activated by the command of the charging control section 510 to produce the power for charging, the secondary battery 540 is supplied with power and is charged. When the constant current circuit 530 is deactivated, the transistor Tr51 is turned off, and thus, the external power is not supplied to the secondary battery 540.
When the external power supply is stopped, the power from the secondary battery 540 is supplied to the DC to DC converter 500 through the diode D52. In this case, the diodes D51, D55 prevent the power from the secondary battery 540 from flowing out of the DC to DC converter 500.
When the secondary battery 540 is charged, a voltage necessary for charging the secondary battery 540 is applied to a positive terminal of the battery through the constant current circuit 530. This voltage is also applied to the DC to DC converter 500 through the diode D52. A maximum voltage necessary for charging a conventional secondary battery such as NiCd batteries is approximately 1.7 times the voltage of the battery. The magnitude of this voltage is not so important, because the DC to DC converter 500 is the step-down-type converter.
In such conventional circuits, the diode D52 is necessary to protect the secondary battery 540 by separating a charging path to the secondary battery 540 from a discharging path from the secondary battery 540. However, when the power from the secondary battery 540 is supplied to the DC to DC converter 500, voltage dropping due to the diode D52 occurs, which reduces an efficiency of a battery use.
The voltage drop across the diode D52 is 0.55 V to 0.7 V. In the electronic apparatus using 6 NiCd batteries, a power loss due to the voltage dropping is 7.6% to 10% of the total battery capacity. In a small sized electronic apparatus using 2 NiCd batteries, the power loss due to the voltage dropping is 23% to 30% of the total battery capacity.
Step-up-type Converter!
FIG. 2 shows a schematic diagram of another conventional charging-and-discharging device with a built-in charging device and the step-up-type converter. The device shown in FIG. 2 comprises a DC to DC converter 600, a charging control section 610 for controlling the charging, a DC connector 620 for connection with the AC adaptor, etc., and a constant current circuit for charging 630 for constructing the charging device. The DC to DC converter 600 includes a DC to DC control section 601. The constant current circuit 530 includes a constant current control section 631. The numeral 640 indicates a secondary battery. Tr61 and Tr62 are transistors. D61-D65 are diodes. F61 and F62 are fuses. L61/L62 is a transformer. L63 is a choke coil. C61 and C62 are capacitors. R61-R63 are resistors.
The DC connector 620 is used when operating the electronic apparatus with the external power source through the AC adaptor, or when charging the built-in secondary battery 640 by supplying the power from the external power source. The DC to DC converter 600 is used to generate the voltage required in the electronic apparatus from the external power source supplied through the DC to DC connector 620 or from the power of the built-in secondary battery 640.
The constant current circuit 630 is supplied with the external power source through the DC connector 620, and generates power required for charging the secondary battery 640 when the charging control section 610 instructs the constant current circuit 630 to charge the secondary battery 640.
The charging control section 610 is a control section for charging the secondary battery 640. The instructions of starting, stopping, and finishing the charging are supplied to the constant current circuit 630 by the charging control section 610.
When the AC adaptor, etc., is connected to the DC connector 620, if the AC adaptor is deactivated for a reason such that the AC power source is not supplied to the AC adaptor, the diode D61 prevents the power of the built-in secondary battery 640 from flowing out. The diode D61 is the back-current-block protection diode.
The diode D62 supplies the power from the secondary battery 640 to the DC to DC converter 600 when no external power source is supplied. Further, when the external power source is supplied through the DC connector 620, the diode D62 is operative as a protection diode which prevents the voltage of the external power source from being applied to the secondary battery 640.
The diode D65 prevents the back current from the secondary battery 640 to the constant current circuit 630.
The constant current circuit 630 is constructed with the conventional switching type regulators, etc. The constant current circuit 630 may use the same configuration as that of the constant current circuit 530 shown in FIG. 1.
The DC to DC converter 600 is a step-up-type switching regulator to raise the voltage of the secondary battery 640 to the voltage required for the electronic apparatus.
When the AC adaptor, etc., is connected to the DC connector 620 to supply the external power source, the external power source is applied to the DC to DC converter 600 through the diode D61, the DC to DC converter 600 producing the voltage required for the electronic apparatus. In this case, the power supplied from the external power source is blocked by the diode D62 and is not applied to the secondary battery 640.
When the external power source is supplied, in a case where the constant current circuit 630 is activated on the command of the charging control section 610 to produce the power for charging, the secondary battery 640 is supplied with the power and is charged. When the constant current circuit 630 is deactivated, the transistor Tr61 is turned off, and thus, the external power is not supplied to the secondary battery 640.
When the external power supply is stopped, the power from the secondary battery 640 is supplied to the DC to DC converter 600 through the diode D62. In this case, the diode D61 prevents the power from the secondary battery 640 from flowing out of the DC to DC converter 600.
When the secondary battery 640 is charged, a voltage necessary for charging the secondary battery 640 is applied to a positive terminal of the secondary battery 640 through the constant current circuit 630. This voltage is also applied to the DC to DC converter 600 through the diode D62. The voltage necessary for charging the conventional secondary battery such as NiCd batteries is approximately 1.7 times the voltage of the secondary battery 640. When several batteries are used in the electronic apparatus, the applied voltage to the DC to DC converter 600 may exceed an output voltage of the DC to DC converter 600. In this case, in the step-up-type DC to DC converter 600 using the choke coil means shown in FIG. 2, an input voltage is directly applied to a load through the choke coil L63 and the rectifying diode D64. Therefore, such DC to DC converter can not be used in the electronic apparatus with the built-in charging device.
For the above case, a step-up-and-down-type DC to DC converter 700 using transformer coupling means shown in FIG. 3 should be used. FIG. 3 shows a schematic diagram of another conventional charging-and-discharging device with the built-in charging device and the step-up-and-down-type converter. The device shown in FIG. 3 comprises the DC to DC converter 700, a charging control section 710 for controlling the charging, a DC connector 720 for connecting with the AC adaptor, etc., and a constant current circuit for charging 730 for constructing the charging device. The DC to DC converter 700 includes a DC to DC control section 701. The constant current circuit 730 includes a constant current control section 731. Numeral 740 indicates a secondary battery. Tr71 and Tr72 are transistors. D71-D75 are diodes. F71 and F72 are fuses. L71/L72 and L73/L74 are transformers. C71 and C72 are capacitors. R71-R73 are resistors.
The device shown in FIG. 3 has the same configuration as that of the device shown in FIG. 2 except that the DC to DC converter 700 is the step-up-and-down-type converter using the transformer coupling means. The constant current circuit 730 may have the same configuration as the circuit 530 shown in FIG. 1.
The step-up-type DC to DC converter 600 using the choke coil means shown in FIG. 2 may have an efficiency of around 80%. However, the efficiency of the step-up-and-down-type DC to DC converter 700 using the transformer coupling means shown in FIG. 3 is limited to around 60%.
As mentioned above, the conventional charging-and-discharging device with the step-up-type converter has the following two problems.
The first problem is that the diode D62 is necessary for protecting the secondary battery by separating the charging path to the secondary battery from the discharging path form the secondary battery. However, when the power from the secondary battery is supplied to the DC to DC converter 600, the power loss due to the voltage dropping across the diode D62 reduces the efficiency of the battery use.
The voltage drop across the diode D62 is 0.55 V to 0.7 V. In the electronic apparatus using 2 NiCd batteries, the power loss due to the voltage drop is 23% to 30% of the total battery capacity.
The second problem is that when the voltage necessary for charging the battery is applied to the positive terminal of the battery through the charging circuit, the voltage may be directly applied to the DC to DC converter 600 through the diode 62.
The voltage necessary for charging the conventional secondary battery such as NiCd batteries is approximately 1.7 times the discharging voltage of the secondary battery. When several batteries are used in the electronic apparatus, the applied voltage to the DC to DC converter may exceed the output voltage of the DC to DC converter. In this case, the DC to DC converter has to be constructed not with the step-up-type converter using the choke coil means but with the step-up-and-down-type converter using the transformer coupling means. The step-up-type DC to DC converter 600 using the choke coil means may have an efficiency of around 80%. However, the efficiency of the step-up-and-down-type DC to DC converter 700 using the transformer coupling means shown in FIG. 3 is limited to around 60%.
B. Over-Discharging Prevention Circuit
Next, a description will be given of conventional circuits for preventing the battery from over-discharging.
In the portable-type electronic apparatus such as note-type personal computers, a variety of batteries such as Li+ (lithium ion) secondary batteries, NiMH batteries, and NiCd batteries are usable. The Li+ battery and the NiMH battery may be easily damaged by over-discharging as compared to the NiCd battery. Once a user over-discharged such batteries by mistake, these batteries are damaged beyond repair. Therefore, to prevent degradation of the battery performance due to the mis-operation by the user, the battery typically includes the over-discharging prevention circuit which shuts off the output of the battery-when the voltage of the battery drops below a given voltage.
To obtain an increased operational time of the electronic apparatus operated with the batteries, the apparatus may mount a plurality of batteries. Such apparatus typically includes a switching circuit for controlling the discharging in an input part of the battery to control discharging order of the battery.
When the above-mentioned apparatus having the switching circuit uses the battery with the built-in over-discharging prevention circuit, a path including the two switching circuits in series is formed and causes a large power loss across the switching circuits.
FIG. 4 shows a schematic diagram of a conventional over-discharging prevention circuit of the battery. In FIG. 4, numerals 800, 800' indicate battery packs. The battery pack 800, 800' includes secondary batteries 810, 810', and control sections 820, 820', preventing the over-discharging, respectively. Numeral 870 indicates an electronic apparatus consuming power, which includes a control section 881 controlling the discharge to the electronic apparatus 870. R80-R89 are resistors, FET81-FET85 are field-effect transistors (FETS) constructing switches, and D81-D83 are diodes.
The electronic apparatus 870 shown in FIG. 4 operates by being supplied with the power from one of the battery packs 800, 800', and an external power supply (DC-IN) through the AC adaptor, etc. The structure of the battery pack 800 is different from that of the battery pack 800'.
The diode D81 is used to prevent the power from the battery from flowing back to the DC-IN side when the power to the apparatus 870 is supplied from the battery pack 800 or the battery pack 800'. The diodes D82 and D83 are used to prevent the power from the DC-IN side from flowing back to the battery pack 800 or the battery pack 800' when the power to the apparatus 870 is supplied from the DC-IN. Specifically, because the transistors FET84 and FET85 have, respectively, a parasitic diode between a source and a drain, back-current-prevention diodes D82, D83 are required.
The transistor FET84 is the switching circuit for controlling the supply of the power from the battery pack 800, which is controlled to be turned on-and-off by the signal from the control section 881. When the transistor FET84 is turned on by the signal from the control section 881, the battery pack 800 is discharged. The transistor FET85 is a switching circuit for controlling the supply of the power from the battery pack 800', which is controlled to be turned on-and-off by the signal from the control section 881 in the same way as the transistor FET 84. When the transistor FET85 is turned on by the signal from the control section 881, the battery pack 800' is discharged.
The resistors R84, 85 are used for detecting a voltage to discriminate whether or not the power is supplied from the DC-IN. The resistors R86, 87 are used for detecting a voltage to discriminate whether or not the power is supplied from the battery pack 800. The resistors R88, 89 are used for detecting a voltage to discriminate whether or not the power is supplied from the battery pack 800'.
In a priority for supplying the power to the apparatus 870, a first priority is the DC-IN, a second is the battery pack 800, and a third is the battery pack 800'. When power supplied from the DC-IN is detected by the resistors R84, R85, the control section 881 controls the transistors FET84, FET85 to be maintained at an off state to stop supplying the power from battery pack 800 and the battery pack 800'. Thus, it prevents consumption of the power of the battery pack 800 and the battery pack 800'.
When a stoppage of the power supply from the DC-IN is detected by the resistors R84, R85, the control section 881 turns on the transistor FET84 to supply the power from the battery pack 800. At this time, to prevent the power of the battery pack 800' from being discharged, the transistor FET85 is maintained at the off state. In this case, if the transistor FET85 in an on state, both the battery pack 800 and the battery pack 800' may simultaneously supply the power to the apparatus 870.
When the resistors R86, R87 detect that the discharging of the battery pack 800 is completed and the secondary battery 810 discharges, the control section 881 turns on the transistor FET85 to supply power from the battery pack 800', and turns off the transistor FET84 to prevent the over-discharging of the battery pack 800. If an over-discharging prevention circuit is combined with the battery pack 800', the above operation is meaningless.
FIG. 5 shows a detail schematic diagram of the over-discharging prevention circuit in the battery pack 800 shown in FIG. 4. Elements R80-R83, FET81, FET82 shown in FIG. 5 correspond to the elements having the same reference numerals shown in FIG. 4. The battery pack 800 further includes comparators IC81-IC84, and reference voltages e81, e82.
The transistor FET81 is the switching circuit for opening the circuit when the secondary battery 810 is in the over-discharging state (the battery voltage is dropped below an given voltage) and when an over current flows from the battery by a short circuit of a load. The transistor FET81 is turned on-and-off by the comparators IC81, IC82. The transistor FET82 is the switching circuit for shutting the circuit to protect the battery when the charging voltage to the secondary battery 810 is increased above a given value. The transistor FET82 is turned on-and-off by the comparator IC83.
The comparator IC81 is used for comparing the battery voltage detected by the resistors R80, R81 with the reference voltage e82. When the battery voltage is dropped by the discharging of the secondary battery 810 below the reference voltage e82, the comparator IC81 produces a low level signal to turn off the transistor FET81 through the comparator IC84. Thus, the comparator IC81 provides an over-discharging prevention circuit.
To simplify the above description of the over-discharging prevention circuit, the circuit IC81 has been considered as a comparator. In practical use, to suppress a fluctuation of an output of the comparator IC81 which is caused by a fluctuation of the battery voltage due to an internal resistance Ri of the battery, the battery pack may include a flip-flop (FF) circuit for storing the signal that the comparator IC81 produces, and reset means for resetting the storage data of the flip-flop circuit when the battery voltage is restored by the charging, etc.
When an over current flows from the battery due to load short-circuiting, the comparator IC82 is used for detecting the over current to protect the battery from the short circuit by opening the circuit. Measurement of the over current is carried out by measuring a dropping voltage across the transistors FET81, FET82 constructing the switching circuit. When the over current flows by the load short-circuiting, a potential at a negative terminal of the battery pack 800 is increased in proportion to the current value by on-resistances of the transistors FET81, FET82. At this time, a potential of the reference voltage e81 is also increased. Therefore, when a current larger than a given value flows, the comparator IC82 comparing the potentials of the reference voltages e81, e82 produces the low level signal, and turns off the transistor FET81 through the comparator IC84.
In the above description, to simplify the description of the over current prevention circuit, the circuit IC82 has been considered as a comparator. In ractical use, when the transistor FET81 is turned off, he output of the comparator IC82 is immediately returned to an high level. To prevent a variation of the output of the comparator IC82, the battery pack may include a flip-flop (FF) circuit for storing the signal that the comparator IC82 produces, and reset means for resetting the storage data of the flip-flop circuit when the short-circuit state is restored.
The comparator IC84 is a three-input comparator. When both of the inputs from the comparators IC81, IC82 are respectively higher than the reference voltage e82, the comparator IC84 produces the high level signal to turn on the transistor FET81. As mentioned above, when the battery is in the over-discharging state or when the over current flows from the battery by the short circuit, the transistor FET81 is turned off to open the circuit by the comparators IC81, IC82, IC84.
The comparator IC83 is an over-voltage detecting circuit to prevent a voltage higher than a specified voltage from being applied to the battery during charging. The comparator IC83 compares the charging voltage measured by the resistors R82, R83 with the reference voltage e81. As a result of the comparison, when the charging voltage is higher than the specified voltage, the comparator IC83 produces the low level signal and turns on the transistor FET82 to shut off current flow in a charging direction.
When the charging voltage is lower than the specified voltage, the comparator IC83 produces the high level signal and turns on the transistor FET82.
When the secondary battery 810 is in the over-discharging state and the transistor FET81 is off, if the charging voltage is applied to the positive terminal of the battery, the transistor FET81 does not intercept the current flow by the parasitic diode, therefore, the battery is charged. When the potential of the battery is raised by the charging of the secondary battery 810, the potential detected by the resistors R80, R81 exceeds the reference voltage e82 and the comparator IC81 produces the high level signal. At this time, the direction of the current flow through the transistors FET81, FET82 is the opposite of that at discharging. Therefore, the potential of the reference voltage e81 does not exceed the potential of the reference voltage e82, and the comparator IC82 also produces the high level signal. As a result, the comparator IC84 turns on the transistor FET81.
C. Plural-Battery-Pack Charging Circuit
Next, a description will be given of a conventional charging circuit mounting two battery packs. In general, based on the output voltage and the power of the battery, the battery pack is constructed with a plurality of battery cells which are connected in series. However, the number of the battery cells in series is limited according to the battery voltage and a power source voltage externally supplied. For example, with NiCd and NiMH batteries, the voltage per cell is 1.2 V, and the charging voltage is approximately 1.7 V. In general devices, a permitted voltage which will not damage a power supply circuit is 16.0 V. Therefore, with NiCd and NiMH batteries, the number of the battery cells in series is limited to 9. With the Li+ (lithium ion) battery, the voltage per cell is maximum 4.2 V. When the permitted voltage for the power supply circuit is 16.0 V, the number of battery cells in series is limited to 3.
Further, the capacity per battery cell is defined by a basic capacity based on battery size. Therefore, to increase the battery capacity, a plurality of groups, each of which has a plurality of battery cells connected in series, have to be connected in parallel.
When charging the secondary battery, it is necessary to control a maximum value of the charging current flowing into each battery. If a current larger than a specified value determined by the battery flows into the battery during charging, the battery is heated. Without an over-heating protection circuit, the battery may explode during the charging.
When the secondary batteries which are connected in series are charged, the same charging current flows through all battery cells. Therefore, by carrying out a constant current control on the charging device side, the battery may always be charged by a current lower than the specified current value.
However, when the secondary batteries which are connected in parallel are charged, the currents flowing through the respective batteries are different according to different impedances of the batteries and different charged or remaining power of the batteries. In a specific example, the charging current may flow only through one of the batteries connected in parallel and no current may flow through other batteries. In this case, the value of the charging current flowing through the one battery is n times the current which regularly flows through the batteries connected in parallel, where n is the number of batteries connected in parallel. Such increase of the charging current is extremely dangerous.
As mentioned above, at present, there is no proper charging method for the secondary batteries which are connected in parallel. Therefore, in general, the batteries connected in parallel are divided into a plurality of battery packs, and each of battery packs is individually charged. As a result, the charging device mounting the plurality of battery packs is proposed.
FIG. 6 shows a schematic diagram of the conventional charging device mounting two battery packs. In FIG. 6, an electronic apparatus includes a charging device 900, a DC connector 901, a DC to DC converter 902, a charging-and-discharging control section 920, and secondary battery packs 930, 940 (which are respectively indicated as a battery A, and a battery B). The charging device 900 has a control section 910 carrying out a constant current control. Tr91, FET91-FET94 are transistors, D91-D95 are diodes, F91, F92 are fuses, L92 is a choke coil, C91 provides a smoothing capacitor, and R90-R94 are resistors.
The DC connector 901 is used when the apparatus is driven by the external power source of the AC adaptor, etc., or when the power is supplied externally of the apparatus for charging the built-in secondary batteries. The DC to DC converter 902 is used for generating the voltage required for the apparatus by the external power source being supplied through the DC connector 901 or by the power being supplied from the built-in secondary batteries.
The charging device 900 is a constant current source for generating power required for charging the secondary batteries when the power is supplied externally through the DC connector 901.
When the AC adaptor is connected to the DC connector 901, if the AC adaptor is in the deactivated state because the AC power source is not supplied to the AC adaptor, etc., the diodes D91 and D95 prevent the power of the built-in secondary batteries from flowing out. The diodes D91 and D95 are the back-current-block prevention diodes.
The diodes D92, D93 supply the power from the secondary batteries to the DC to DC converter 902 when no external power source is supplied. Further, when the external power source is supplied through the DC connector 901, the diodes D92, D93 are operative as the protection diodes which prevents the voltage of the external power source from being applied to the secondary batteries.
When the power from the AC adaptor, etc., is not supplied through the DC connector 901, and the power from the secondary batteries is supplied to the DC to DC converter 902, the transistor FET93 and FET94 are operative as switching circuits for controlling whether the battery A or the battery B supplies the power. In a case where the power from both the batteries A and B is simultaneously discharged, the transistors FET93, FET94 are not necessary. The transistors FET91, FET92 are switching circuits for selecting a battery to be charged, and controls the flow of the charging current supplied from the charging device 900 either through the battery A or through the battery B.
The charging device 900 is the constant current circuit for charging, and comprises the main switching transistor Tr91, the choke coil L92, the flywheel diode D94, the smoothing capacitor C91, the sense resistors R90-R94 for controlling the current, and the control section 910. The constant current circuit for the charging 900 may be constructed with the conventional switching-type regulators, etc.
When the AC adaptor, etc., is connected with the DC connector 901 and external power is supplied, the external power is applied to the DC to DC converter 902 through the diode D91, wherein the DC to DC converter 902 produces the voltage required for the apparatus. In this case, the power supplied from the external power source is interrupted by the diodes D92, D93, and is not applied to the battery packs 930, 940 of the secondary batteries.
When external power is supplied, and when the constant current circuit for charging operates to generate the power for the charging, the secondary battery may be charged. When the constant current circuit is deactivated, the circuit is shut by the transistor Tr91, and, thus, the battery may not be charged.
When the external power supply is stopped, the power from the battery packs 930, 940 of the secondary batteries are supplied through the transistors FET93, FET94 and the diodes D92, D93. A source electrode of the transistor FET93 is connected to an anode side of the diode F92, and a drain electrode thereof is connected to the positive terminal side of the battery A. A gate electrode of the transistor FET93 is connected to a control signal from the charging-and-discharging control section 920. In the same way, a source electrode of the transistor FET94 is connected to an anode side of the diode F93, and a drain electrode thereof is connected to the positive terminal side of the battery B. A gate electrode of the transistor FET94 is connected to the control signal from the charging-and-discharging control section 920.
An internal parasitic diode of the transistor FET93 is oriented in a forward direction from the drain side to the source side. Therefore, the transistor FET93 operates as the switching circuit for the current flow from the battery A, but is maintained at the on state for the current flow into the battery A. To prevent a back current to the battery A, the diode D92 is connected in series to the transistor FET93. In the same way, an internal parasitic diode of the transistor FET94 is oriented in a forward direction from the drain side to the source side. Therefore, the transistor FET94 operates as the switching circuit for the current flow from the battery B, but is maintained at the on state for the current flow into the battery B. To prevent a back current to the battery B, the diode D93 is connected in series to the transistor FET94.
When the charging-and-discharging control section 920 detects that the input power supply from the DC-IN is stopped, the control section 920 turns on the transistor FET93 or the transistor FET94 to urge the battery to discharge. When the discharging from the battery A is instructed, the charging-and-discharging control section 920 turns on the transistor FET93 by setting the gate electrode of the transistor FET93 to a ground potential. When the discharging from the battery B is instructed, the charging-anddischarging control section 920 turns on the transistor FET94 by setting the gate electrode of the transistor FET94 to the ground potential.
The diode D91, D95 prevents power from the batteries A, B from flowing out of the DC to DC converter 902.
When the external power is supplied by the AC adaptor and the constant current circuit for the charging in the charging device 900 is operating, the power generated in the charging device 900 charges the battery A or the battery B through the transistor FET91 or FET92 for the switching. When the battery A is charged, the transistor FET91 is turned on to close the current path to the battery A. At this time, since the transistor FET92 is in the off state, all current from the constant current circuit is used for charging the battery A.
When the battery B is charged, the transistor FET92 is turned on to close the current path to the battery B. At this time, since the transistor FET91 is in the off state, all current from the constant current circuit is used for charging the battery B.
FIG. 7A shows a charging flowchart for the control section 910 and the charging-and-discharging control section 920 shown in FIG. 6. FIG. 7B shows a discharging flowchart for the control section 910 and the charging-and-discharging control section 920 shown in FIG. 6.
In the charging process shown in FIG. 7A, when it is detected that external power is supplied to the DC-IN (S21), the following processes are carried out. The charging-and-discharging control section 920 turns off the transistors FET93, FET94 (S22), and then, turns on the transistor FET91 and turns off the transistor FET92 (S23). On the other hand, the control section 910 turns on the transistor Tr91 to charge the battery A (S24). In a step S25, when the battery A is fully charged, and completion of the charging is detected, the process proceeds to step 26. In step S26, the transistor Tr91 is turned off. After that, the transistor FET91 is turned off, and the transistor FET92 is turned on (S27). And next, the transistor Tr91 is turned on to charge the battery B (S28). In step S29, when the battery B is fully charged, and completion of the charging is detected, the transistor Tr91 is turned off (S30).
In the discharging process shown in FIG. 7B, in step S41, the transistor FET93 is turned on, and the transistor FET94 is turned off to discharge the power from the battery A. When the potential of the battery A is lowered, and full discharge of the power in the battery A is detected in step 42, the transistor FET93 is turned off and the transistor FET94 is turned on to proceed to the discharging of the battery B in step S43. In step S44, when full discharge of the power in the battery B is also detected, a low-battery processing in the apparatus is started in a step 45.
In the conventional electronic apparatus provided with the plurality of battery packs shown in FIG. 6, the battery pack to be charged is selected by the switching circuit. After the charging for the first selected battery pack is completed, the next battery pack is selected to be charged. At present, such a serial charging is generally used.
In this way, when the plurality of battery packs are serially charged in the single charging circuit, a charged battery and an non-charged battery are not connected in parallel. Therefore, the switching transistors FET93, FET94 are required for controlling the discharging of each battery, and also the back-current-prevention diodes D92, D93 are required for preventing the charging and the discharging between the charged battery and the un-charged battery.
As a result, during the discharge of battery, a power loss due to resistivity of the switching transistors FET93, FET94, and a power loss due to a voltage drop across the back-current-prevention diodes D92, D93 is caused. These power losses reduce the operational efficiency of the battery.
There is a method for reducing the power loss by using FETs instead of the back-current-prevention diodes D92, D93. In the method, the transistors for switching and the FETs for the diodes are connected in series such that the internal parasitic diodes of the two different transistors are connected to each other in an opposite direction. However, the power loss due to the resistivity of the two FETs occurs.
Further, the method has a disadvantage that a cost of the FET is greater than twice the cost of the diode.
Also, since the plurality of battery packs are serially charged, the batteries can not be discharged in parallel except when the charging of all the batteries is completed. Therefore, even if a plurality of battery packs are provided, the batteries are discharged one by one. Thus, the maximum current value which can be discharged is restricted.
FIG. 8 shows a schematic diagram of a conventional charging apparatus having the same number of charging devices as batteries.
The apparatus shown in FIG. 8 includes charging devices 950, 950', a DC connector 951, a DC to DC converter 952, and secondary battery packs 970, 980 (which are referenced to batteries A, B). The charging devices 950, 950' respectively have control sections 960, 960' for carrying out the constant current control. Tr92 is a transistor, D90, D96-D99 are diodes, F93, F94 are fuses, L93 is a choke coil, C92 provides a smoothing capacitor, and R95-R99 are resistors.
The charging device 950 is used for charging the battery A, and the charging device 950' is used for charging the battery B. In this way, by providing a plurality of charging devices, charging time for the plurality of batteries can be reduced. However, to prevent a charging current to the battery A from flowing into the battery B, the back-current prevention diode D98 is required. In the same way, to prevent a charging current to the battery B from flowing into the battery A, the back-current prevention diode D97 is required.
As a result, when the battery is discharged, a voltage drop across the back-current prevention diodes D97, D98 occurs, which causes a power loss due to the drop. Therefore, the power loss reduces the operational efficiency of the battery. Further, there is a disadvantage that the number of circuits is increased due to an increase of the number of the charging devices.
(2) Problems of The Prior Art
A. Voltage Converter
a) To separate the current path to the battery during charging from the current path from the battery during discharging, the diode has to be provided. The diode caused a power loss due to the voltage drop across the diode.
For example, when the electronic apparatus operates with two NiCd batteries, the power loss is approximately 20 to 25% of the total battery power.
b) In the apparatus including the step-up-type DC to DC converter using the choke coil, the conventional charging circuit can not be adapted to the apparatus. Therefore, the DC to DC converter is required to be changed to the step-up-and-down-type DC to DC converter using the transformer coupling means shown in FIG. 3. However, though the efficiency of around 80% is achieved in the step-up-type DC to DC converter using the choke coil, the efficiency in the step-up-and-down-type DC to DC converter using the transformer coupling means is limited to around 60%. In this converter, an effective usage of the battery is not obtained.
B. Over-Discharging Prevention Circuit
In the electronic apparatus mounting the plurality of battery packs, to prevent the respective batteries from discharging simultaneously, the output sections from the respective batteries need respective switching circuits for controlling the on-and-off state of the discharging circuit. However, when the over-discharging prevention circuit is equipped with the battery pack to prevent the degradation of the battery due to over discharging, the over-discharging prevention switching circuit in the battery pack and the discharging control switching circuit are connected in series. Therefore, the power loss across the switching circuits is doubled.
C. Plural-Battery-Pack Charging Circuit
a) To charge the plurality of batteries, the switching circuits for switching the current flow at the charging and the back-current prevention diodes are inserted. However, the switching circuits and the diodes cause power losses due to the voltage drops across the switching circuits and the diodes at the discharging from the battery.
For example, when the electronic apparatus operates with two NiCd batteries, the power loss is approximately 20 to 25% of the total battery power.
There is the method for reducing the power loss by using the FETs instead of the back-current prevention diodes. However, the transistor for the switching circuit and the FET for the diode need to be connected in series. Therefore, the power loss for two FETs occurs. Further, the method has the disadvantage that the cost of the FET is approximately twice the cost of the diode.
b) Since the plurality of batteries are serially charged in the single charging device, the total charging time for the batteries is increased according to the number of batteries.
c) To carry out the parallel charging for reduction of the total charging time for the batteries, the same number of charging devices as the batteries is necessary.
d) Since the discharging from the plurality of batteries is serially carried out, the maximum current value which can be discharged is restricted. To increase the maximum discharging current value, parallel discharging from the batteries is required. However, since the charging of the batteries is serially carried out, it is difficult to perform parallel discharging.